In the aerobic biological sludge treatment of waste water, mirco-organisms (called bacteria) are encouraged to feed on organic material in the waste water in the presence of oxygen (air). The products of this process are carbon dioxide, water, and additional bacteria. Since this process requires oxygen, it is usually carried out in a vessel called a reaction basin, where the mixture of waste water and bacteria are aerated. After aeration, the sludge mass (bacteria) are separated from the waste water by settling in a device called a clarifier. The purified waste water is discharged as treated effluent while some of the activated sludge is recycled back to the reaction basin for re-use.